Conventionally, protection for electric induction motors against overheating has been achieved through the use of overcurrent elements, which are also used for fault protection for electric power systems. The overcurrent elements are typically coordinated with the time/current characteristic curves of the particular motor being protected. These time/current curves are typically available from the manufacturer of the motor. However, such coordination is usually valid thermally only for limited operating conditions, since the time overcurrent elements do not model the actual thermal characteristics, i.e. the actual heating and cooling, of the motor during its operation. Further, the reset characteristics of overcurrent elements do not bear any relation to the actual operating thermal time constant of a motor.
Typically, the overcurrent elements will be set with a time delay long enough to allow the motor to start, i.e. they will permit a required high level of current for a sufficient time to start the motor, but shorter than the thermal limit time for a locked rotor portion of the motor. A separate negative sequence overcurrent element is often used to prevent thermal damage caused by excessive unbalanced three phase current. Unbalanced current, which produces negative sequence current, in turn produces a severe heating effect in the rotor. The negative sequence elements are themselves typically set quite sensitively, but in structure and operation are independent of and do not take into account the heating effect of the positive sequence current component of the unbalanced three phase current.
As indicated above, however, overcurrent elements do not operate in accordance with the temperature rise in the motor.
One solution to the disadvantages of the above-described conventional motor protection system involves a "thermal model" approach, which takes into account both the positive and the negative sequence heat sources at work on the motor. In this solution, set out in U.S. Pat. No. 4,914,386 to Zocholl, motor voltage and current values are directly measured at the motor terminals, and those values are used to calculate the change in the impedance at the motor terminals during the motor start-up period. From this information, the speed of the motor may be determined, and motor speed is indicative of the actual thermal condition of the motor. The relevant portions of that patent, concerning the general development of a thermal model for an induction motor, are hereby incorporated by reference.
The thermal model approach of the '386 patent, however, has disadvantages, since a voltage measurement is necessary, which requires additional hardware elements, such as voltage transformers. Hence, such a system is expensive. Further, in many cases certain required motor parameter information to implement the thermal models is not readily available from the motor manufacturer. Hence, existing thermal model approaches are basically impractical, except in a few motor situations, even though the thermal model approach generally is advantageous, since it is related to the actual thermal conditions in the motor.
In a somewhat different approach, temperature sensors have been positioned within the structure of the motor itself in order to sense motor temperature directly. However, these attempts have been largely unsuccessful, since it is difficult to properly locate such temperature sensors without interfering with the operation of the motor. Also, such an approach has not heretofore been very effective because by the time the threshold of the temperature sensor is reached, substantial damage to the motor has often already occurred.
Hence, there remains a need for thermal protection system for motors which is accurately reflective of the actual thermal conditions within the motor, yet is practical for a wide variety of motors and responsive to actual thermal conditions prior to any damage occurring in the motor.